This invention relates to air mixers for mixing together two different air flows, particularly an air mixer for an air distribution system suitable for a building or other similar structure.
In air handling systems designed for large buildings such as office towers and other large structures, there has been a need to mix together at least two different air flows before distributing the mixed air flow throughout the air ducts of the building by means of a fan. Although a number of air mixers have been developed for bringing together and mixing two different air streams, often these air mixers are not very efficient and/or they require a substantial amount of space in the building in order to function properly. The two air streams that often must be mixed in an air handling system are generally return air that is coming back from the building itself and fresh outside air. In cold weather, the return air will normally be quite warm, for example, room temperature, while the outside air can often be quite cold.
In these air handling systems for buildings, air stratification that results from the momentum inherent in moving air streams can keep air streams of different temperatures from mixing for quite some distance. This in turn can cause the air handling system to operate poorly or inefficiently and can also result in poor indoor air quality. During the winter time, lack of proper mixing of the incoming air streams can result in freezing or damage of heating coils that are part of the heating system and can generate control sensor errors. During the summer, poor mixing of the air streams can result in the lack of proper control of the indoor air temperature and can increase the energy consumption of the air conditioning system. The heat transfer capacities at the cooling coils are based on airflow at uniform temperature and velocity across the coils. A non-uniform temperature distribution for the entering air will cause reduced heat transfer at the coils and the desired temperature in the building may not be maintained.
Moreover, the problems caused by poor mixing of air streams are becoming more serious as the amount of outdoor air is increased in the air distribution system. It is noted that government regulations and building users are now often requiring a greater amount of outdoor air. An increased amount of air is now being required by IAQ standards such as ASHRAE Standard 62.
Various solutions have been proposed in the past to prevent air stratification in an air handling system and to prevent the damage that it can cause to the system. For example, glycol additives have been used to prevent frozen heat transfer coils. Although such additives may prevent frozen coils, they do not prevent the problem of reduction in heat transfer capacity of the coils due to uneven air temperature of the entering air. Dampers and high velocity jets have also been used to help in the mixing of two or more air streams but often the use of such devices creates unacceptable levels of pressure drop in the system. Specially designed air mixers have also been proposed in the past and these can improve the mixing of the air streams. However, these known mixers have some inherent defects which can be caused by the air streams being forced to pass through a narrow cross-section of the mixer. These known air mixers generally require more downstream space, can create a non-uniform downstream velocity profile and can cause a high pressure drop across the mixer. In addition, a non-uniform velocity profile caused by the air mixer can generate an extra pressure drop at downstream filter and coil sections.
An early form of air mixer is shown and described in U.S. Pat. No. 1,395,938 issued Nov. 1, 1921 to P. Barducci. In this mixer, two different air streams enter the casing of the mixer at an angle of about 90 degrees to one another. A number of boxes are arranged across the width of the air duct formed by the casing and these boxes open into an inlet duct at the side of the casing. The boxes are arranged side-by-side and are spaced apart from each other. All the boxes are provided with mouths that are open in the direction of the air flow. A main incoming air flow passes between these boxes and creates a suction effect at the mouths of the boxes so as to draw air in through the side inlet and into the downstream end of the casing where the two air streams are mixed.
More recent U.S. Pat. No. 5,463,967 issued Nov. 7, 1995 to Airflow Sciences Corporation describes a static mixer designed for use with a coal-fired power plant. The mixer has a series of parallel walls arranged in side-by-side spaced apart relationship to form a series of rectangular spaces. The perimeters of these spaces are selectively closed to define respective first and second inlets and an outlet. The mixer creates interleaving of the two air streams and thus promotes increased homogeneity some distance downstream of the confluence of the streams. This known mixer also has turning vanes for turning one of the sub-divided streams as it passes through the mixer.
It is an object of the present invention to provide an improved air mixer that can help avoid undesirable air stratification in the plenum of an air distribution system and that at the same time has low pressure drop.
It is a further object of the present invention to provide an air mixer for an air distribution system that can be manufactured at a reasonable cost and that is highly efficient.